The present invention relates to an image forming apparatus which transfers a toner image formed on an image carrier onto a transfer sheet.
In a conventional image forming apparatus of an electrophotographic process, and in particular, in an image forming apparatus having a transfer means for transferring a toner image electrostatically, an image carrier, a sheet feeding roller located to precede a transfer area (hereinafter referred to as a registration roller) and a heating and rotating body which heats and fixes a toner image on a transfer sheet (hereinafter referred to as a fixing roller), a buckle of transfer sheet in a feeding path affects image quality, especially there is a defect that the buckle generates a jitter of an image (hereinafter referred to as a transfer jitter), which has been a problem. It is presumed that this defect occurs more easily in a transfer means wherein a close contact of a transfer sheet in a transfer area is not so satisfactory like a corona transfer, than in a transfer means wherein a close contact of a transfer sheet is better in a transfer area, like a roller transferring or a belt transferring.
Generally, the buckle of a sheet in a feeding path is caused by a speed difference between a linear speed of a fixing roller and that of a registration roller located to precede a transfer area of the image carrier, in cases where the length of a transfer sheet is longer than the length of the transfer sheet path from the nip position of the registration roller to the nip position of the fixing roller. And the buckle occurs when the linear speed of the fixing roller is relatively slow. However, in order to make a higher close contact between the image carrier and the transfer sheet in the transfer area, an appropriate buckle of the transfer sheet gives good results, further, decreases a crease caused by a pressure contact of a transfer roller.
If this buckling amount becomes greater than a reasonable amount, the buckling is released just after a trailing edge of a sheet passes through the registration roller, and the transfer sheet goes backward against the image carrier at the transfer area, and therefore, for example, a transfer jitter (hereinafter referred to as a trailing edge transfer jitter), which is shaped likely to be compressed, occurs near a trailing edge of the sheet in the feeding direction of the transfer sheet. Especially, this tendency is observed in the image forming apparatus in which the transfer sheet is conveyed vertically near the transfer area.
Further, in order to prevent the crease from occurring on the sheet, there may be given a difference between a diameter of a central section and that of both ends of a roller, and in this case, the diameter of the central section of the roller is made smaller. Accordingly, the linear speed of the central section of the fixing roller is lower than that of the both ends of the fixing roller. When the sheet, whose width is narrower corresponding to the center part of the roller, is used, the difference between the linear speed of the fixing roller and that of the registration roller becomes large to increase the buckling amount, thus, there is a tendency that the trailing edge transfer jitter occurs easily.
Further, when the sheet width is the same for various types of the transfer sheets, the longer the length of the sheet is, the greater the total amount of the buckling becomes, resulting a disadvantage for the trailing edge transfer jitter.
On the other hand, when the rotating speed of the registration roller is changed, there is a fear that a lengthwise magnification of an image will go wrong. Accordingly, it is necessary to reduce the influence on the lengthwise magnification of the image, as well as to control the occurrence of the transfer jitter.
It is an object of the present invention to provide an image forming apparatus in which the transfer jitter of the trailing edge of the image can be prevented from occurring as far as possible, even when a transfer sheet size varies, and in which errors of the image lengthwise magnification are less.
The object of the invention is attained by the following structures.
Structure (1) An image forming apparatus having a image reading device which reads an image by scanning a document, a writing exposure device which forms the image as a latent image on an image carrier, a developing device which forms a toner image by developing the latent image, a registration roller which conveys a transfer sheet to a transfer area, and is driven by a driving source being different from that for the image carrier, and a rotating heat body which heats and fixes the transfer sheet on which the toner image is transferred, wherein, there is provided a control means which changes and controls rotating speed of the registration roller according to the sizes of the transfer sheets, and changes and controls reading and scanning speed for the document.
Structure (2) An image forming apparatus having a writing exposure device which forms an image as a latent image on an image carrier, a developing device which develops the latent image into a toner image, a registration roller which feeds a transfer sheet to a transfer area, and is driven by a driving source being different from that for the image carrier, and a heating and rotating body which heats and fixes the transfer sheet on which the toner image is transferred, wherein, there is provided a control means which changes and controls rotating speed of the registration roller according to the sizes of the transfer sheets, and changes and controls writing speed onto the image carrier.
Structure (3) An image forming apparatus having a writing exposure device which forms an image as a latent image on an image carrier, a developing device which develops the latent image into a toner image, a registration roller which feeds the transfer sheet to a transfer area, and is driven by a driving source being different from that of the image carrier, and a rotating heat body which heats and fixes the transfer sheet on which the toner image is transferred, wherein, there is provided a control means which changes and controls rotating speed of the registration roller according to the sizes of the transfer sheets, and changes and controls rotating speed of the image carrier and rotating speed of the heating and rotating body.